1. Field of the Invention
The present invention relates to a case member of an airbag device that accommodates an airbag between the case member and a cover member, the airbag being inflated and expanded from a retracted state through the introduction of gas.
2. Description of the Related Art
Conventionally, for example, there has been known an airbag device provided in a steering wheel of a vehicle such as an automobile. Such an airbag device includes a back plate serving as a case member, an inflator mounted on the back plate, a retainer, an airbag, a cover member or the like.
The back plate uses a metal material. A steel plate is folded and is subject to a welding or deep drawing (press) process, or is subject to mold shaping of a light alloy (magnesium or aluminum), so that an open back plate shaped like a case is obtained.
However, since the back plate formed using the steel plate is heavy and the back plate formed using the light alloy requires high material cost and process cost and needs thickness setting or rib setting for obtaining strength, it is difficult to achieve sufficient lightness.
Therefore, in recent years, in order to achieve lightness, miniaturization, low cost or the like of an airbag device, there has been known an airbag device in which an entire back plate is formed of synthetic resin (for example, refer to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-505462 (page 5 and FIG. 1)). However, even in this configuration, since setting of ribs or the like is required in order to guarantee sufficient strength tolerant to the expansion of an airbag, it is not easy to achieve sufficient lightness. Furthermore, the back plate made of synthetic resin may be easily deformed (e.g., warped), after being pulled out from a mold, by internal residual stress in a forming process.
Meanwhile, there has also been known an airbag device in which most of a back plate is formed using a steel plate or the like and only a partial complicated molded component, in which a support body for allowing the back plate and a boss portion (a boss plate) to be engaged with each other, is disposed, is formed using synthetic resin (for example, refer to Japanese Laid-open Patent Application No. 2011-255753 (pages 4 to 6 and FIGS. 7 and 8)). However, in this configuration, it is possible to save a cost for a burr-finishing process or the like in a bending process and die-cast forming of the complicated molded component, but there is no large difference relative to an airbag device in which an entire back plate is formed using a steel plate, in terms of lightness.
As described above, light weight, low-cost, and stable dimension are demanded as a component used for a back plate of an airbag device.